The surface quality of a flexible magnetic disk as measured by the error free reproduction of recorded data can be substantially improved by mechanically treating the disk surface. The desirability of surface treatment and its advantages were recognized when it was discovered that untreated disk surfaces had reduced errors after they had been used for a period of time. There was rapid improvement for an initial period of use. After this, the disk remained essentially unchanged until the effect of mechanical wear became evident. Following this discovery, efforts were made to speed up the break-in period since it was not practical to run the disks in normal fashion for the time required to improve their performance.
The various disk manufacturers use a variety of techniques, not all of which are publicly known. Examples of surface treatment are shown in U.S. Pat. Nos. 3,943,666 and 4,179,852 and IBM Technical Disclosure Bulletin Vol. 14, No. 5, October 1971 pages 1144-1146; Vol. 19, No. 10, March 1977 pages 3829-3830 and Vol. 20, No. 3, August 1977 pages 1146-1147.
These techniques are variously referred to as burnishing, polishing or honing but the objective, removal of asperities to improve the smoothness of the disk, is the same regardless of the name applied to the technique.
U.S. Pat. No. 3,943,666 describes a process which uses an abrasive cylinder to burnish a flexible disk supported against a resilient backing plate. The surface deviation finish claimed for this process is 1.8 AA in a 12-22 second treatment. While surface finish dimensions are a factor, the real test is how the disk performs in actual practice. Even a very smooth finish may not perform well if the finish is obtained by a process which creates voids in the recording media.
U.S. Pat. No. 4,179,852 uses an abrasive loaded pad to polish a flexible disk. While the patent does not describe the quality of the surface finish obtained, it does specify a burnishing period of approximately 20 seconds.
U.S. Pat. No. 3,486,274 describes an apparatus for grinding the surface of a rigid disk. The metallic grinding pads are fastened to a rotating head in a fashion which permits them to conform to the undulations in the rigid disk. The grinding pads have slots of varying widths according to their distance from the center of the rotating head. Nothing is said concerning the ability to finish flexible disks.
IBM Technical Disclosure Bulletin Vol. 14, No. 5, pages 1444-5 shows a flexible disk surface treatment which uses a sharp flat hard blade which bears against a flexible disk in a region backed by a resilient pad. The treatment is specified to take 20 seconds.
IBM Technical Disclosure Bulletin, Vol. 19, No. 10, pages 3829-30 illustrates a burnishing process which typifies the prior art. A cup type burnishing tool is positioned within a hole in a flat Bernoulli plate which supports a rotating flexible disk. The edge of the cup forms a circular knife which engages the disk and smoothes out any high spots or bumps. The flow of air outwardly across the knife edge burnishing cup appears to create an area of reduced pressure within the cup to draw the flexible disk into engagement with the cup.
IBM Technical Disclosure Bulletin, Vol. 20, No. 3, August 1977, pages 1146-7 sets forth an improved version of the process described in Vol. 19. The improvement consists of a vacuum applied to the interior of the burnishing tool and a change to a rounded edge on the tool instead of the sharp edge previously described. This improvement is directed specifically to media which uses chromium dioxide coatings.
From the wide range of processes described above it can be seen that the treatment is more of an art than a science. Where one process uses a sharp edged tool another uses a round edge. Some processes use abrasives either bonded to a tool or in a slurry and others use only a tool. Suffice to say that what works for one media is often not satisfactory for another or at least does not produce an optimum result.